Since the issuance of U.S. Pat. No. 3,028,365 in Apr. of 1962, aromatic polycarbonate has become well known and accepted as a thermoplastic resin suitable for a wide variety of uses including injection molding, extrusion and film formation. The chemistry, synthesis, properties and applications of these polycarbonates are extensively discussed in Chemistry and Physics of Polycarbonates by Schnell, Interscience, 1964 and Polycarbonates by Christopher and Fox, Reinhold, 1962.
Although polycarbonates have some inherent flame resistance, being self-extinguishing, ever more demanding flame retardancy requirements have spawned numerous attempts to increase this property. Two general approaches have been followed.
One approach has been to add substantial amounts of halogen, particularly bromine or chlorine, to polycarbonate compositions. The halogen can be carried by polycarbonate polymer chains as in U.S. Pat. Nos. 3,751,400 and 3,334,154 or by a monomeric compound as in U.S. Pat. No. 3,382,207. However, the presence of substantial amounts of halogen has been found to be detrimental to the properties of the polycarbonate and numerous additives such as those proposed in U.S. Pat. Nos. 3,647,747 and 3,733,295 have been proposed to overcome these detrimental effects.
An alternative approach has been the addition of various organic and/or inorganic metal salts to impart the desired flame retardancy. U.S. Pat. No. 3,775,367 teaches the use of about 0.01 to 1 wt. % of perfluoroalkane sulfonic acid salts of alkali metals and suggests the presence of halogen either on the polymer backbone or by additive compound will enhance the flame retarding efficiency of these salts. U.S. Pat. No. 3,836,490 teaches the use of 0.00005 to 0.1 wt. % of selected organic alkali salts of carboxylic acids in halogen free or halogen bearing polycarbonates wherein the salts are soluble in the polycarbonate melt. In contrast, German Published Pat. No. 2,149,311 teaches the use of about 0.05 to 3.0 wt. % of insoluble alkali metal salts, particularly salts of inorganic acids, phosphonic acids and sulfonic acids. U.S. Pat. No. 3,535,300 teaches the use of small amounts of specified metal salts (which do not include alkali metal salts) in combination with about 0.01 to 1 wt. % of halogen carried on the polymer backbone or on an additive. The halogen is said to "synergistically" interact with the salt. U.S. Pat. No. 4,110,299 discloses the use of small amounts of additives selected from the group consisting of organic alkali metal salts and organic alkaline earth metal salts in combination with additives selected from the group consisting of an inorganic halide and an organic monomeric or polymeric aromatic or heterocyclic halide to improve the flame retardancy of an aromatic polycarbonate. The effect of the organic salt and the inorganic or organic halide is said to be a synergistic interaction.
In either the halogen or the salt approach, two aspects of flame retardancy must be considered. The UL subject 94 test which has come to be widely accepted for testing the flame resistance of organic polymers measures both a polymeric resistance to being consumed by a flame and a polymer's tendency to drip flaming particles when exposed to a flame. The later aspect, dripping, has been found to be the more difficult to control. U.S. Pat. No. 3,876,580 teaches the use of 2 to 6 wt. % of glass fibers in combination with either between 1 and 3 wt. % of halogen (chlorine or bromine) or between 0.01 and 1 wt. % of salts of nickel or alkali metals to achieve both properties of flame retardance. The patent notes that if the salt and halogen are both added, less of each is required to obtain the desired flame retardance. German Pat. (Offenlegungsschrift) No. 2,535,262 teaches adding fluorinated polyolefins such as polytetrafluoroethylene to a polycarbonate containing organic alkali metal salts to retard dripping. Finally, U.S. Pat. No. 4,110,299 suggests the addition of a fluorinated polyolefin, fibrous glass or a siloxane in combination with an organic salt/inorganic or organic halide mixture to diminish an aromatic polycarbonate's tendency to drip.
All of the above approaches have been tried and found to meet to some extent the three primary objectives of flame retardancy in thermoplastic resins:
(1) Increasing the polymers resistance to fire as measured both by composition and dripping; PA1 (2) Minimizing the detrimental effects of the flame retardant, be it halogen bound to the polymer backbone or additives or both, on the other properties of the polymer, particularly mechanical properties and high temperature stability such as resistance to degradation during molding; and PA1 (3) Using the least expensive system which will meet the other two objectives.
Clearly these three objectives may be somewhat contradictory and a balance must be sought. For instance, a sufficient amount of some additives may achieve any desired degree of flame retardance, but at an unacceptable loss of other polymer properties.
The flame retardance of a polycarbonate may be increased by increasing either the amount or efficacy of the flame retardant system. The former approach is less attractive because as the level of either chemically incorporated halogen or additive is increased, the polymers other properties, particularly mechanical properties, are usually decreased.
The efficacy of a flame retardant system may be increased by lowering the melt flow rate (and therefore increasing the relative viscosity and molecular weight) of the polycarbonate. Lower melt flow resins generally have better mechanical properties and are able to tolerate higher additive levels. But more importantly, lower melt flow resins have an inherently greater resistance to dripping which, as discussed above, is one of the two aspects of flame retardancy.
Unfortunately, lowering the melt flow of the resin has disadvantages. As the melt flow is reduced, it becomes more difficult to process the polycarbonate into finished form, particularly by injection molding which is one of the most important end uses. In addition, the processing of lower melt flow polycarbonate usually requires the resin to be subjected to higher temperatures for longer periods of time thus heightening the possibility that the resin will suffer some degradation during processing.
Conversely the detrimental effects of a flame retardant system may be decreased by lowering its amount or by using a system of which the polycarbonate is more tolerant. The amount of flame retardant may be reduced by using a system which is more effective, e.g. substituting bromine for chlorine (as taught in Canadian Pat. No. 725,726). The less of a flame retardant that is present usually the more tolerant polycarbonate is of it although certain flame retardants such as combinations of iron salts and bromine have been found to cause particular problems of high temperature stability.
Naturally, the least expensive flame retardant system which achieves both the desired flame retardant properties and the desired mechanical properties is to be preferred. Usually, the more readily available or the more easily synthesized flame retardants are somewhat less expensive. For instance, the organic alkali metal carboxylates of U.S. Pat. No. 3,836,490 are less expensive and more readily available than the alkali metal perfluoro alkane sulfonates of U.S. Pat. No. 3,775,367.
However, increasingly more stringent requirements have made it ever more difficult to meet both criteria 1 and 2. Many of the prior art flame retardant systems are unable to achieve required flame retardance and still retain adequate mechanical properties. For instance, higher degrees of flame retardance, e.g. 94V-O in UL subject 94, are being required in thinner sections, e.g. 1.6 mm. As the section thickness is decreased, the difficulty of achieving the required flame retardance increases.
An objective of the present invention is to find a flame retardant system for an aromatic polycarbonate which meets the stringent requirements of both criteria 1 and 2 above without the use of a halogenated organic salt, specifically a halogenated organic alkali metal salt or a halogenated organic alkaline earth metal salt.